For liquid permeable covering layers for absorbent articles of this kind, which during use are intended to be in contact with the body of the user, there are high demands for softness as well as dryness.
It has, however, proven difficult to achieve a liquid permeable covering layer with a soft and textile-like surface which remains dry even after repeated wetting, when the covering layer is used on an absorbent article.
The use of non-woven fibrous fabrics, so-called non-woven materials, for creating a soft and smooth surface on an absorbent article has been known for a long time. Providing the non-woven material with apertures in order to more rapidly transfer liquid through the surface material down to an underlying absorbent material layer, is also known. When making the apertures, openings are created in the material, which are larger than the space between the fibres in the non-woven material.
One such apertured non-woven material is previously known from EP 235,309. The apertured non-woven material consists of a spunlace material with a high content of hydrophobic fibres. In a spunlace process, holes are made in a fibrous material by treating the material with water jets which are sprayed against the material at very high pressure. The spunlace material constitutes one of two layers in a surface material laminate and is intended to be the layer which, during use, is closest to the user. The purpose is that the liquid will be guided through the holes in the spunlace material down to the underlying layer. The spunlace material has a higher content of hydrophobic fibres than the underlying material layer in the surface laminate. The fibres of the uppermost spunlace layer consist of 70% hydrophobic fibres and 30% hydrophilic fibres, while the underlying material layer consists of equal amounts of hydrophobic and hydrophilic fibres. Thus, the underlying layer thus has the ability to drain liquid from the upper layer.
However, a problem with the material described in EP 235,309 is that holes which are formed by water jets become irregular both in size and shape, and have fibres which protrude from the edges of the holes around and in the holes. These protruding fibres decrease the area of the holes, and also, due to capillary effects, wick liquid into the material between the holes. The protruding fibre ends and the irregular size and shape of the holes significantly increase the risk of liquid remaining in the layer after wetting. Since a very small amount of liquid is sufficient for the surface material to be perceived as wet, this is of course a significant disadvantage of the known surface material.
A similar surface material is described in EP 272,683. This publication also concerns an apertured covering layer of non-woven material. In the vicinity of the holes, which have been formed by perforating the non-woven material, there are relatively loose fibres which are intended to function as transport canals for the liquid down to an underlying non-woven layer of the so called meltblown type. As long as the fibres of the perforated layer are arranged in such a way that they lead the liquid towards the underlying layer, the described surface material works. It is, however, a well known fact that a non-woven material consists of irregularly formed fibres which are difficult to arrange in any particular direction. This means that fibre capillaries which are intended to transport liquid to the underlying layer will also spread liquid in a horizontal direction across the surface of the non-woven material. For this reason, some of the liquid will remain in the surface material after wetting, and the user of an article provided with the surface material will experience the surface of the article as being wet and unpleasant against the skin.
A further problem with the above described non-woven materials is that it is difficult to achieve a particular, well defined, hole size. It is, for instance from EP 409, 535 well known that the dimension of the holes in a perforated material is crucial in order to achieve optimal permeability for the liquid. For non-woven materials having some areas with a dense fibre structure and other areas with a sparse fibre structure, this means that it is difficult to obtain a uniform hole size. This is due to the fact that the holes in the dense fibre areas become smaller, since they are surrounded by more fibres.
Further, the perforated non-woven materials in previously known surface materials further have a relatively low tensile strength, since the described hole-making process weakens the material. Since it is important that the strength of the material is such that there is no risk that it will break, either during the hole-making process, during the manufacturing of the absorbent article, or during use of the finished absorbent article, evidently the weakening of the known materials is a problem.
In EP 214,608 holes are made in a non-woven material using warm needles which heat the non-woven material to a temperature which is slightly below the melting point of the material. The holes which are thus made in the material are surrounded by an edge which has a densified fibre structure. With a material with holes made in this way, the above mentioned problems with varying hole sizes and weakening of the material have been solved to a certain extent. However, the problem of avoiding liquid being spread out in the non-woven material and remaining in its fibre structure still remains. The denser fibre structure surrounding the holes is intended to absorb liquid thereby wicking the liquid further through the holes in to an underlying material layer. There is, however, a risk that some liquid will remain in the denser hydrophilic fibre structure around the holes. Liquid can also spread horizontally in the plane of the non-woven material via the fibre capillaries in the non-woven material. Since the non-woven material during use is in direct contact with the skin of the user, this is of course extremely undesireable.